1. The Field of the Invention
The present invention generally relates to optical wave guides and optical wave guide amplifiers for use in optical networks. More specifically, the present invention relates to packaging for optical waveguide amplifiers.
2. The Relevant Technology
Optical networks are widely used to communicate data over short and long distances in various networks, including telecommunications and data networks. Optical networks using optical fibers have become a preferred way of transmitting data due to the high bandwidth inherent in optical networks, as well as the decreasing cost of components of such networks. In optical networks, data is encoded in optical signals, which are transmitted over optical fiber between nodes in the network. Optical transceivers are used to convert electrical signals to optical signals and vice versa and to transmit and receive optical signals that are propagated over the optical network.
When optical signals are sent over long distances, they may need to be repeated or amplified to ensure the signals do not degrade to the point of data loss. Such losses result in slower overall transmission times and lower data rates in a given band. There are both theoretical and practical limits to the data speeds that can be obtained in a fiber optic network.
Light traveling through different media is reflected and refracted at each interface where the index of refraction changes. The amount of light deflected and the directions of deflection depend on the angle of incidence with the interface and the refractive indices of the media across the interface. For example, approximately 4% of the light traveling from glass into air and approaching the interface at a normal direction is reflected backward into the glass along the same normal direction of incidence. The remaining 96% proceeds, in this case with unchanged direction, into the air.
One component of an optical network where light passes from one media to another is an optical planar waveguide. An optical waveguide is a light conduit having a slab, strip or cylinder of dielectric material surrounded by another dielectric material having a lower refractive index. Optical planar waveguide technology has been widely used to build components in optical communication networks, such as switches, amplifiers, modulators, MUXs, DEMUXs, etc.
FIGS. 1A and 1B show an existing planar waveguide for an amplifier. Wave guide 10 is made by sandwiching a first dielectric material 14 between two sheets of a second dielectric material 12. A beam of light 16 entering the first dielectric material 14 is constrained within this material by the second dielectric material 12.
Given that the light is traveling in the z direction, the two normalized modes for the propagating light are linearly polarized waves with polarization along the x and y directions. The x-polarized mode is called transverse electric (TE) polarization, while the y-polarized mode is called the transverse magnetic (TM) polarization. Polarization dependent loss (PDL) in the propagating light or signal is caused because the TE and TM modes travel with different propagation conditions in the planar waveguide. The result is a large PDL, which is unacceptable in most planar waveguide devices.
A technique to reduce the PDL is shown in FIG. 2. An input light beam 110 is split into two orthogonal polarization beams 112, 114 by a birefringence crystal beam displacer 116. The polarization of one of the beams 112 is rotated 90° after passing through a λ/2 waveplate 118 so that both beams 112, 114 have identical polarization. Then both of the beams 112, 114 are launched into a waveguide 120 as TE waves. Upon exiting the waveguide 120, the polarization of one of the beams 114 will rotate 90° after passing through a second λ/2 waveplate 122 such that the two beams 112, 114 again have orthogonal polarizations. The two beams 112, 114 will then be re-combined into one beam 126 by a second birefringence crystal beam displacer 124.
Unfortunately, the technique uses an in-line configuration where light beam 110 enters waveguide 120 from one side and exits from the opposite side of waveguide 120. This is difficult to implement in situations where a reduction in PDL is desired in a non-in-line situation.